a:hover img {
opacity: .5;
-webkit-opacity: .5;
-moz-opacity: .5;
filter: alpha(opacity=50);
-ms-filter: “alpha(opacity=50)”;
}

a img {
-webkit-transition: opacity 1s ease-out;
-moz-transition: opacity 1s ease-out;
-ms-transition: opacity 1s ease-out;
transition: opacity 1s ease-out;
}